Magnetic fluids used in technical applications, commonly referred to as ferrofluids, are a dispersion of finely divided magnetic or magnetizable particles ranging between thirty (30) and one hundred fifty (150) angstroms in size and dispersed in a liquid carrier.
The magnetic particles are typically covered with surfactants or a dispersing agent. The majority of industrial applications using magnetic fluids incorporate iron oxides as magnetic particles. The most suitable iron oxides, for magnetic fluid applications, are ferrites such as magnetite (Fe.sub.3 O.sub.4) or ferric oxides (Fe.sub.2 O.sub.3) such as gamma. Ferrites and ferric oxides offer a number of physical and chemical properties to the magnetic fluid, the most important of these being saturation magnetization, viscosity, magnetic stability, and chemical stability of the whole system. The amount of magnetic particles in the magnetic fluid composition can range up to 40% by volume.
The surfactants have two major functions. The first is to assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal's force and magnetic interaction, and the second is to provide a chemical composition on the outer layer of the covered particle which is compatible with the liquid carrier and the chemicals in the surrounding environment. Most of tne magnetic fluids employed today have one (1) to three (3) types of surfactants arranged in one (1), two (2) or three (3) layers around the magnetic particles. The surfactants, for magnetic fluids, are long chain molecules having a chain length of at least sixteen (16) atoms such as carbon, or a chain of carbon and oxygen, and a functional end group at one end. The functional end group can be of a cationic, an anionic or a nonionic nature. The functional end group is attached to the outer layer of oxides (magnetic particles) by either chemical bonding or physical force or a combination of both, and the chain or tail of the surfactant provides a permanent distance between the particles and compatibility with the liquid carrier. For all practical purposes, the amount of surfactant in the magnetic fluid composition can range up to thirty (30)% by volume.
The carrier is generally an organic molecule, either polar or non polar, of various chemical composition such as hydrocarbon (polyalpha olefins, aromatic chain structure molecules), esters (polyol esters), silicone, or fluorinated and other exotic molecules with a molecular weight range up to five thousand (5,000).
There are several physical and chemical properties of the magnetic fluid related to the type of carrier such as viscosity, evaporation rate, resistance and compatibility with the surrounding environment.
There are many patents related to the preparation of magnetic fluids and the most relevant of which for this invention are:
U.S. Pat. No. 3,531,413 describes a process where magnetic particles are initially dispersed in a non-polar solvent, and then flocculated with a polar solvent whereby the particles are separated from the initial solvent and resuspended in a different solvent.
U.S. Pat. No. 3,917,538 describes a process which consists of grinding coarse magnetic particles in an aqueous carrier using a dispersing agent. The aqueous ferrofluid obtained from the grinding process is flocculated and the magnetic particles are separated out of the aqueous solution. The particles are then washed, dried and resuspended in an organic carrier using a second dispersant.
U.S. Pat. No. 3,700,595 describes using a carboxylic acid having at least a twelve (12) carbon chain as a surfactant which is oil soluble and water insoluble, or a high molecular weight polyisobutene carboxylic acid surfactant.
U.S. Pat. No. 4,280,918 describes a process for preparation of a magnetic dispersion for use in magnetic coating. The magnetic particles are coated with a uniform material, preferably colloidal silica. The coating prevents aggregation of magnetic particles. The pH of the slurry is adjusted to between three (3) and six (6), by an acid, to produce a positive electrostatic charge on the magnetic particles and to mix a colloidal silica having a negative electrostatic charge. The two oppositely charged particles are attracted to and the silica particles are irreversibly bonded to the magnetic particles.
U.S. Pat. No. 4,315,827 describes a method of preparing a stable ferrofluid composition by dispersing magnetic particles in polyphenyl ether using surfactants with one functional polar group reactive with the surface of the particles, and a tail group containing phenyl, benzyl or phenoxy groups soluble in the liquid carrier.
U.S. Pat. No. 4,356,098 describes a method of preparing a stable silicone oil ferrofluid composition which comprises a colloidal dispersion of finely divided magnetic particles in a liquid silicone oil carrier, a dispersing amount of silicone oil surfactants containing a functional group which forms a chemical bond with the surface of magnetic particles, and a tail group which is soluble in the silicone oil carrier to provide a stable magnetic composition. The tail group of the surfactant has a number of atoms of silicon and oxygen chains, or siloxane, in order to be soluble in the silicone oil.
U.S. Pat. No. 4,430,239 describes a stable ferrofluid composition comprising a colloidal dispersion of finely divided magnetic particles in a liquid carrier, and a dispersing amount of a dispersing agent, which agent comprises an acid phosphoric acid ester of a long chain alcohol, the long chain alcohol being compatible with the polar liquid carrier.
U.S. Pat. No. 4,576,725 describes a method of preparing a magnetic fluid by dispersing metallic magnetic particles, having an average diameter of several hundreds of angstroms, in a base liquid. The particles are obtained by condensation of metallic vapor in the liquid carrier. The metal magnetic particles in the ferrofluid are oxidized very rapidly. The oxidation process of the metallic particles will dramatically change the initial property of the ferrofluid.
U.S. Pat. No. 4,599,184 describes an attempt to improve the oxidation and magnetic stability of the magnetic metal particles obtained from metallic vapor condensation by coating the particles with a surface active agent or surfactant. In order to obtain a stable magnetic fluid, the particles have to be covered with a surfactant as in any other process, to obtain a stable magnetic fluid.
U.S. Pat. Nos. 4,604,229 and 4,687,596 describe methods for producing stable electrically conductible magnetic fluids using cationic high molecular weight surfactants and polar carriers.
U.S. Pat. No. 4,608,186 describes a magnetic fluid comprising fine metal particles of cobalt, and a surface active agent selected from a group consisting of polyglycerine fatty acid esters, sorbitan fatty acid esters and a mixture thereof. The liquid carrier is a hydrocarbon. The composition contains tocopherol as an antioxidant additive.
U.S. Pat. No. 4,624,797 describes a magnetic fluid comprising fine particles of cobalt, and a surface active agent selected from the group consisting of oil soluble anionic sulfosuccinate and nonionic polyglycerine fatty acid ester or the group consisting of polyethyleneglycol alkyl ether and a low volatility solvent medium.
Metallic magnetic particles of a diameter less than two hundred (200) angstroms and evenly coated with a surfactant are highly unstable and oxidized very rapidly. Today, there are no commercial applications of such fluid using magnetizable metal particles. The major drawback of this process is the oxidation of the magnetic particles.
U.S. Pat. No. 4,938,886 describes a super paramagnetic fluid comprising magnetic particles; a dispersing agent of a formula A-X-B anchored to the magnetic particles, wherein A is derived from a nonionic surface active agent precursor having a terminal OH group, the precursor being selected from a group consisting of ethoxylated or propoxylated alcohols and other ethoxylated compounds, B is a carboxylic acid group which anchors the dispersing agent to the magnetic particles and X is a connecting group between A and B; and a carrier liquid which is a thermodynamically good solvent for A.
U.S. Pat. No. 5,013,471 describes a magnetic fluid where the particles are covered with a chlorosilane surfactant having a chain with ten (10) to twenty-five (25) atoms of carbon. Fluorine atoms are substituted for the hydrogen atoms of the hydrocarbon chain of the chlorosilane surfactant and fluorinated oil is used as a carrier. There is no other surfactant used in this process. According to this reference, the surfactant chlorosilane has to be large enough to disperse the particles and to assure the colloidal stability of the magnetic fluid by providing sufficient distance between the particles.
One object of the present invention is to use a silane surface modifier of very low molecular weight, e.g. one (1) to ten (10) carbon atoms, in the tail chain to be able to penetrate between the existing surfactant to cover the free (exposed) surface which is not covered by the large molecular weight surfactant. According to the present invention the silane can not be used to disperse the magnetic particle alone.
U.S. Pat. No. 5,064,550 describes a super paramagnetic fluid which is a stable colloid comprising a non-polar hydrocarbon carrier, and the magnetic particles are coated with at least one acid selected from the group consisting of an organic acid containing only carbon and hydrogen atoms in the chain connected to the carboxyl group where the chain contains at least nineteen (19) carbon atoms and an amino acid acylated with the fatty acid, wherein the organic and amino acids are branched, unsaturated or both, and an ashless polymer is provided to increase the viscosity of the super paramagnetic fluid.
U.S. Pat. No. 5,085,789 describes a ferrofluid composition consisting essentially of fine particles of ferromagnetic particles with alkylnaphtalene being used as the carrier and a surfactant with the hydrophobic portion consisting of alkylnaphtalene structure.
U.S. Pat. No. 5,124,060 describes a ferrofluid composition consisting essentially of an organic solvent carrier, ferromagnetic particles coated with oleophilic groups exhibiting an affinity for said organic solvent, and a fluorocarbon surface active material.
U.S. Pat. No. 5,143,637 describes a magnetic fluid consisting of ferromagnetic particles dispersed in an organic solvent, a low molecular weight dispersing agent, and an additive with a carbon number between twenty-five (25) and fifteen hundred (1,500). The low molecular weight dispersing agent is used to disperse the particles in an organic carrier. In the summary of this reference there is a discussion about using a coupling agent, such as silane, as a dispersant. However, the coupling agent has to have a large enough molecular weight to perform as a dispersant. It should be mentioned that, in U.S. Pat. No. 5,143,637, there is no particular disclosure claim directed to using silane as an additive or even as a dispersant. The thermal stability of the fluid is increased by adding a high molecular weight additive, e.g. up to twenty thousand (20,000), such as polystyrene, polypropylene, polybutene, or polybutadiene polymers.
U.S. Pat. No. 5,147,573 describes a method of preparing a colloidal dispersion of electrically conductive magnetic particles consisting essentially of superparamagnetic particles, an electrically conductive organo metallic compound, a dispersing agent comprising a nonionic, an anionic or a cationic surfactant, and a hydrocarbon organic solvent.
U.S. Pat. No. 4,554,088 employs polymeric silane as a coupling agent. The coupling agents are a special type of surface active chemicals which have functional groups at both ends of the long chain molecules. One end of the molecule is attached to the outer oxide layer of the magnetic particles and the other end of the molecule is attached to a specific compound of interest in those applications, such as drugs, antibody, enzymes, etc.
U.S. Pat. No. 5,240,628 describes a process for producing a magnetic fluid, which comprises adding a solution of N-polyalkylenepolyamine-substituted alkenylsuccinimide in a water-insoluble or water-sparingly-soluble organic solvent to an aqueous suspension of fine particles of ferrites and stirring the resulting mixture, thereby forming an emulsion and absorbing the N-polyalkylenepolyamine-substituted alkenylsuccinimide onto the fine particles of ferrites, then distilling off water and the organic solvent therefrom and dispersing the fine particles of N-polyalkylenepolyamine-substituted alkenylsuccinimide-absorbed ferrites in a base oil of low vapor pressure having a vapor pressure of not more than 0.1 mm Hg at 25.degree. C.
In none of the above discussed patents is there an attempt to cover the surface area of the magnetic particles which is not already covered by the large size surfactants.